Nanoporous separators for batteries and related manufacturing methods

ABSTRACT

Provided is a lithium battery, wherein the battery comprises an anode, a cathode, wherein the cathode comprises one or more transition metals, an electrolyte, and a porous separator interposed between the cathode and anode, wherein the separator comprises an anionic compound. Also provided are methods of manufacturing such batteries.

RELATED APPLICATIONS

This application claims the benefit of and priority to U.S. ProvisionalPatent Application No. 62/231,539, filed Jul. 9, 2015. This applicationis also a continuation-in-part of and claims priority to U.S. patentapplication Ser. No. 15/130,660, filed Apr. 15, 2016. The contents ofeach of the above-referenced applications are incorporated herein byreference in their entireties.

STATEMENT OF GOVERNMENT RIGHTS

This invention was made with government support under Grant NumberDE-EE00054333 awarded by the U.S. Department of Energy. The governmenthas certain rights in the invention.

FIELD OF THE INVENTION

The present invention generally relates to the field of batteries andother electric current producing cells, such as capacitors andlithium-ion capacitors. More particularly, the present inventionpertains to separators for lithium batteries and related manufacturingmethods.

BACKGROUND OF THE INVENTION

Throughout this application, various patents are referred to by anidentifying citation. The disclosures of the patents referenced in thisapplication are hereby incorporated by reference into the presentdisclosure to more fully describe the state of the art to which thisinvention pertains.

Lithium batteries are widely used in portable electronics, such assmartphones and portable computers. Among the new applications forlithium batteries are high power batteries for hybrid, plug-in hybrid,and electric vehicles. However, broad acceptance of electric vehiclesrequires batteries that can be constructed at lower cost and thatinclude improved safety features.

Existing processes for manufacturing lithium batteries, includingrechargeable and non-rechargeable lithium batteries, and other types ofbatteries, are relatively slow, complex and expensive. For example,rechargeable lithium-ion batteries are typically constructed byinterleaving strips of the various layers of the battery to form astack. These layers may include a plastic separator, a conductive metalsubstrate with a cathode layer coated on both sides, another plasticseparator, and another conductive metal substrate with an anode layercoated on both sides. This interleaving is usually done on manufacturingequipment that is inefficient and costly to construct and operate. Thus,there is a need for manufacturing techniques that do not requireinterleaving discrete battery layers.

As noted above, current lithium batteries are fabricated using metalsubstrates. During manufacture, these metal substrates are typicallyslit into discrete battery stacks. This has been known to result inmetal fragments being embedded into the separator or other portion ofthe finished battery, which can lead to a short circuit, or otherdangerous condition. Thus, there is a need for improved manufacturingtechniques that eliminate these safety concerns.

In addition, one of the known challenges in reducing the cost oflithium-ion batteries is the composition of the cathode. In this regard,the cathode material often comprises thirty percent, or more, of thetotal battery cost. Thus, there has been increased interest in utilizingmanganese and its oxides as a cathode material because manganese isconsiderably less expensive than other cathode materials and is found inabundance in nature. However, when used as a cathode in lithium-ionbatteries, manganese is easily dissolved, particularly at highertemperatures. During operation, the dissolved manganese ions aredeposited on the separator and anode resulting in reduced battery cyclelife. Moreover, this migration problem is not limited to manganese. Inthis regard, there has also been a shift in the battery industry tocathodes comprising nickel-manganese-cobalt oxide (NMC) and, inparticular, nickel-rich NMC. However, nickel and cobalt ions, likemanganese ions, diffuse through the separator and onto the anode,reducing battery cycle life. Thus, it would be advantageous if themigration of these metals (e.g. manganese, nickel and cobalt) could becontrolled and eliminated.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a battery stack orbattery that could be fabricated on less complex, less expensive andhigher speed automated processing equipment than, for example, theequipment utilized for portable computer batteries. Another object isprovide a battery that is less expensive to make than existingbatteries, and which could utilize transition metals, such as manganese,nickel and cobalt, but control the migration of these metals, withoutreducing battery cycle life.

The present invention meets the foregoing objects through the batterystacks and batteries described herein. The battery stacks and batteriesdescribed herein include various coatings and materials, which aredescribed below. Examples of batteries to which the present inventionapply include a single electric current producing cell and multipleelectric current producing cells combined in a casing or pack. One suchtype of battery is a lithium battery, including, for example,rechargeable or secondary lithium ion batteries, non-rechargeable orprimary lithium metal batteries, rechargeable lithium metal batteriesand other battery types such as rechargeable lithium metal alloybatteries.

The battery stacks described herein include a separator, electrode andcurrent collector. A battery stack comprising a positive electrode incombination with a battery stack comprising a negative electrode,together, form a battery. The battery stacks and batteries describedherein include a separator to keep the two electrodes apart in order toprevent electrical short circuits while also allowing the transport oflithium ions and any other ions during the passage of current in anelectrochemical cell. Examples of separators that may be utilized inlithium batteries include ceramic separators and polyolefin separators.Ceramic separators include separators comprising inorganic oxides andother inorganic materials.

The battery stacks and batteries described herein include an electrodethat comprises electroactive material. The electrode layer may beconfigured to function as the anode (negative electrode) or cathode(positive electrode). In a lithium ion battery, for example, electriccurrent is generated when lithium ions diffuse from the anode to thecathode through the electrolyte. Examples of electroactive materialsthat may be utilized in lithium batteries include, for example, lithiumcobalt oxide, lithium manganese oxide, lithium iron phosphate, lithiumnickel manganese cobalt oxide (NMC), and sulfur as electroactivematerials in the cathode layers and lithium titanate, lithium metal,silicon, lithium-intercalated graphite, and lithium-intercalated carbonas electroactive materials in the anode layers.

These battery stacks and batteries described herein also include acurrent collector, which can be one or more current collection layersthat are adjacent to an electrode layer. One function of the currentcollector is to provide a electrically conducting path for the flow ofcurrent into and from the electrode and an efficient electricalconnection to the external circuit to the cell. A current collector mayinclude, for example, a single conductive metal layer or coating, suchas the sintered metal particle layer discussed herein. As discussedfurther below, an exemplary conductive metal layer that could functionas a current collector is a layer of sintered metal particles comprisingnickel, which can be used for both the anode or cathode layer. Inembodiments of the invention, the conductive metal layer may comprisealuminum, such as aluminum foil, which may be used as the currentcollector and substrate for the positive electrode or cathode layer. Inother embodiments the conductive metal layer may comprise copper, suchas a copper foil, which may be used as the current collector andsubstrate for the negative electrode or anode layer.

The batteries described herein also include an electrolyte, such asthose that are useful in lithium batteries. Suitable electrolytesinclude, for example, liquid electrolytes, gel polymer electrolytes, andsolid polymer electrolytes. Suitable liquid electrolytes include, forexample, LiPF₆ solutions in a mixture of organic solvents, such as amixture of ethylene carbonate, propylene carbonate, and ethyl methylcarbonate.

In one embodiment the present invention includes a lithium batterycomprising: an anode, a cathode, wherein the cathode comprises one ormore transition metals, an electrolyte, and a porous separatorinterposed between the cathode and anode, wherein the separatorcomprises an anionic compound. In one embodiment the anode compriseslithium metal. In one embodiment the cathode comprises one or moretransition metals selected from the group consisting of manganese,nickel and cobalt. In one embodiment the porous separator comprises oneor more inorganic oxides or nitrides. In one embodiment the separatorcomprises boehmite. In one embodiment the anionic compound comprises twoor more anionic groups. In one embodiment the anionic groups areselected from the group consisting of sulfonate and carboxylate. In oneembodiment the cation of the anionic groups comprises a lithium ion. Inone embodiment the anionic compound comprises greater than about 0.1weight percent of the weight of the porous separator. In one embodimentthe anionic compound is an anthraquinone. In one embodiment the anioniccompound is a photosensitizer. In one embodiment the photosensitizer isan oxygen scavenger. In one embodiment the separator comprises polymerformed by the absorption of photons by the photosensitizer. In oneembodiment the anionic compound is an oxidizer of lithium metal. In oneembodiment the porous separator has an average pore diameter of lessthan about 100 nm. In one embodiment the porous separator is a scavengerof HF in the electrolyte. In one embodiment the porous separatorinhibits the migration of transition metal cations through theseparator. In one embodiment the cathode and the anode compriseelectrode layers and one or more electrode layers are coated on theseparator.

In one embodiment the present invention includes a separator for anelectric current producing cell comprising: one or more inorganic oxidesor nitrides, a binder, and an anionic compound. In one embodiment theanionic compound is selected from the group consisting of sulfonate andcarboxylate. In one embodiment the anionic compound is an oxidizer oflithium metal dendrites. In one embodiment, the anionic compound is aphotosensitizer.

In one embodiment the present invention includes a battery stackcomprising: a porous separator, an electrode layer adjacent the porousseparator, a current collector layer coated on the electrode layer, anda reinforcement area along one or more edges of the battery stack,wherein the reinforcement area comprises a polymer. In one embodimentthe reinforcement area comprises a polymer impregnated in the pores ofthe porous separator. In one embodiment the reinforcement area comprisesa layer of polymer overlying the porous separator. In one embodiment theporous separator further comprises a photosensitizer.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of the present disclosure will be more fullyunderstood with reference to the following, detailed description whentaken in conjunction with the accompanying figures, wherein:

FIG. 1 is a cross-sectional view of a partially assembled battery stack1 showing a porous separator 20 coated over a substrate 10 and releasecoating 30.

FIG. 2 is a cross-sectional view of the battery stack of FIG. 1, withthe addition of electrode lanes 40 a, 40 b coated over the porousseparator layer 20.

FIG. 3 is a plan view of the battery stack shown in FIG. 2.

FIG. 4 is a cross-sectional view of the battery stack of FIGS. 2 and 3,with the addition of a current collector layer 50 coated over theelectrode lanes and reinforcement portions 52 coated over separatorlayer 20.

FIG. 5 is a plan view of the battery stack of FIG. 4, with the additionof conductive tabbing patches 60 on reinforcement portions 52, andfurther indicating the location of slit lines S₁, S₂ and S₃.

FIG. 6 is a plan view of the battery stack assembly shown in FIG. 5after a slitting step has been performed.

DETAILED DESCRIPTION

This invention pertains to battery stacks for use in batteries, such aslithium ion batteries and lithium metal batteries, as well as methods ofmaking such batteries and related nanoporous separators. Among otherbenefits, the coated battery stacks and batteries of the presentinvention have a lower cost and provide improved safety.

The present invention includes, but is not limited to, the followingdesigns for lithium batteries and stacks and methods of making suchbatteries. In the following examples, the coated stack may be either ananode stack or a cathode stack, depending on the electrode materialselected.

One aspect of the present invention will be described with reference toa process for manufacturing a lithium battery. One suitable process isdescribed in co-pending U.S. patent application Ser. No. 15/130,660,which is incorporated by reference in its entirety. The process mayutilize a reusable substrate 10, onto which the various layers of thebattery stack are coated. Once the battery stack is assembled, thebattery layers (e.g., electrode, separator, current collector) aredelaminated from the substrate 10 and the substrate can be reused tocreate another battery stack according to the same process. The use of areusable substrate provides cost saving benefits and reduces waste.However, it is noted that this same process can be carried out using adisposable or non-reusable substrate.

The first step of the process includes coating a substrate 10 with arelease coating 30. The substrate 10 and release coating 30 will bereferred to herein, collectively, as the release layer. The substrate 10may comprise any strong, heat resistant film, such as polyethyleneterephthalate (“PET”), polyethylene-naphthalate (“PEN”) or otherpolyester film. In a preferred embodiment, the substrate 10 may comprisea 75-125 μm thick PET film. PET provides a robust substrate for thedisclosed process since it has a high tensile strength, and ischemically, thermally and dimensionally stable. Advantageously, as aresult of the thickness, tear resistance and resistance to distortion ofPET film, wide rolls, such as those having a width of 1.5-2.0 meters,can be processed quickly and reliably. For example, coated batterystacks can be processed at speeds of 125 m/min.

A heat stable and compression resistant porous separator layer 20 isthen coated onto the release layer. The coated separator layer 20 can bemade thinner than known free-standing separators. The coated separatorlayer 20 is also highly compatible with a roll-to-roll coating process,such as that described above.

In one embodiment, the separator layer is coated at a thickness of 5-8μm across the full width of the release film. FIG. 1 shows an example ofa cross-sectional view of the assembly 1 after the coating of theseparator 20 onto the substrate 10 and release coating 30.

Examples of a suitable separator layer 20 for the present inventioninclude, but are not limited to, the porous separator coatings describedin U.S. Pat. Nos. 6,153,337 and 6,306,545 to Carlson et al., U.S. Pat.Nos. 6,488,721 and 6,497,780 to Carlson and U.S. Pat. No. 6,277,514 toYing et al. Certain of these references disclose boehmite ceramicseparator layers, which are suitable for use with the instant invention.See, e.g., U.S. Pat. No. 6,153,337, Col. 4,11. 16-33, Col. 8,11. 8-33,Col. 9,1. 62-Col. 10,1. 22 and Col. 18,1. 59-Col. 19,1. 13; U.S. Pat.No. 6,306,545, Col. 4,1. 31-Col. 5,1. 17 and Col. 10,11. 30-55; and U.S.Pat. No. 6,488,721, Col. 37,11. 44-63. U.S. Pat. No. 6,497,780 disclosesboehmite ceramic separator layers, as well as other ceramic separatorlayers including those with a xerogel or sol gel structure, all of whichare suitable for use with the instant invention. See, e.g., U.S. Pat.No. 6,497,780, Col. 8,1. 66-Col. 10,1. 23 and Col. 11,1. 33-Col. 12,1.3. U.S. Pat. No. 6,277,514 teaches coating one or more protectivecoating layers onto a boehmite ceramic separator layer. These protectivecoating layers include inorganic layers designed to protect the metalanode surface, such as in a lithium metal anode. See, e.g., U.S. Pat.No. 6,277,514, Col. 5,1. 56-Col. 6,1. 42, Col. 9,11.14-30, Col. 10,11.3-43, Col. 15,11. 27-56 and Col. 16,11. 32-42.

Preferred separator layers suitable for use with the present inventionare also described in U.S. Pat. App. Pub. No. 2013/0171500 by Xu et al.One such separator comprises a microporous layer comprising (a) at least50% by weight of an aluminum boehmite and (b) an organic polymer,wherein the aluminum boehmite is surface modified by treatment with anorganic acid to form a modified aluminum boehmite. See, e.g., Pars. 28,and 34-36. The organic acid may be a sulfonic acid, preferably an arylsulfonic acid or toluenesulfonic acid, or a carboxylic acid. Themodified boehmite may have an Al₂O₃ content in the range of 50 to 85% byweight, or more preferably in the range of 65 to 80% by weight. Theseparator may comprise 60 to 90% by weight of the modified aluminumoxide, or more preferably 70 to 85% by weight of the modified boehmite.In embodiments of the invention, the microporous layer may be a xerogellayer. The organic polymer may comprise a polyvinylidene fluoridepolymer. The separator layer 20 may further comprise a copolymer of afirst fluorinated organic monomer and a second organic monomer.

Other preferred separator layers suitable for use in embodiments of thepresent invention are described in International App. No. WO2014/179355by Avison et al. The separator layers described in that applicationinclude boehmite, a variety of other pigments, and blends thereof. See,e.g., WO2014/179355, Pars. 4-6, 8, 21, 26, and 27. In a preferredembodiment, the separator layer 20 is a nanoporous inorganic ceramicseparator. More specifically, the nanoporous battery separator includesceramic particles and a polymeric binder, wherein the porous separatorhas a porosity between 35-50% and an average pore size between 10-90 nm,or more preferably between 10-50 nm. The ceramic particles may beinorganic oxide particles or inorganic nitride particles. Preferably,the porous ceramic separator exhibits less than 1% shrinkage whenexposed to a temperature of 200° C. for at least one hour. The ceramicparticles may include at least one of Al₂O₃ or alumina, AlO(OH) orboehmite, AlN, BN, SiN, ZnO, Zr0₂, SiO₂, or combinations thereof. In apreferred embodiment, the ceramic particles include between 65-100%boehmite and a remainder, if any, of BN. Alternatively, the ceramicparticles may include between 65-100% boehmite and a remainder, if any,of MN. The polymeric binder may include a polymer, such aspolyvinylidene difluoride (PVdF) and copolymers thereof, polyvinylethers, urethanes, acrylics, cellulosics, styrene-butadiene copolymers,natural rubbers, chitosan, nitrile rubbers, silicone elastomers, PEO orPEO copolymers, polyphosphazenes, and combinations thereof.

In one embodiment, the separator layer comprises an inorganic oxide thatis surface modified by treatment with an organic sulfonic acid to form amodified inorganic oxide and also comprises an inorganic oxide that isnot surface modified by treatment with an organic sulfonic acid. In oneembodiment, the organic sulfonic acid is an aryl sulfonic acid, and morepreferably a toluenesulfonic acid. In one embodiment, the inorganicoxide comprises a boehmite. In one embodiment, the boehmite is surfacemodified by treatment with an organic sulfonic acid to form a modifiedhydrated aluminum oxide. In one embodiment, the blend of the treated andthe non-treated inorganic oxides is in a ratio of about 1:3 to 3:1 byweight, preferably in the range of about 2:3 to 3:2. In one embodiment,a crosslinking material, such as an isocyanate, is added to provideadditional mechanical strength to the separators. Multifunctionalisocyanates are preferred. The weight percent of the isocyanate istypically in the range of 1 to 10% of the weight of the polymer in theseparator with about 6% by weight being a preferred level.

In one embodiment, a water extraction is done on the separator to removeany water-soluble materials from the separator, such as an organicsulfonic acid. This water extraction is, preferably, done before theseparator is used in the construction of a battery stack. One method forperforming such water extraction is utilizing a water bath immersion atabout 80° C., which is highly compatible with the high speedmanufacturing process discussed above. Furthermore, at the time ofimmersion, an anionic treatment (discussed further below) can be appliedto the separator.

Among other benefits, water extraction increases the percent porosity ofthe separator, which provides better ionic conductivity of theelectrolyte and greater mechanical strength to the separator, for agiven percent porosity. For example, some of the organic sulfonic acidthat is not covalently bonded, or otherwise strongly bound, to theinorganic oxide can be removed by water extraction. For example, aseparator comprising an inorganic oxide treated with an organic sulfonicacid may exhibit a weight loss of about 1% upon water extraction. Thisweight reduction is sufficient to increase the percent porosity of theseparator by 2 to 3% and increase the mechanical strength of theseparator by 10% or more.

In one embodiment of this invention, the separator is dried at atemperature of at least 130° C. up to a temperature of as high as 240°C., preferably under vacuum conditions. In one embodiment, this dryingat high temperatures is done for more than 1 hour, and preferably morethan 3 hours. In one embodiment, a vacuum drying is done in a dry cellcomprising electrodes and separator prior to filling the cell withelectrolyte. This high temperature drying of the separators, especiallyunder vacuum conditions, is useful for increasing the mechanicalstrength of the separators, typically by at least 10%, and for reducingany residual water content that may degrade the battery, such as informing any HF and other decomposition products of the electrolyte saltby reaction with water. This reduction in water content in turn causesless dissolution of the nickel, manganese, and other transition metalsin the cathodes to prevent their undesirable diffusion through theseparator into the anode. This reduction in water content by hightemperature drying, enabled by the very high heat stability of theceramic separators, also contributes to a more efficient formationcycling of the battery to form the anode and cathode stabilizationlayers, commonly referred to as the solid electrolyte interface (SEI)layers (discussed further below), which provides longer cyclinglifetimes and better rate capability.

As the lithium-ion battery industry moves toward the use of transitionmetal cathodes, such as NMC and others with higher nickel and manganesecontent, it is desirable to inhibit the diffusion and migration ofdissolved metal ions (e.g., nickel and manganese ions) into and throughthe separator and to the anode. In one example, a lithium-ion batterywas constructed with a porous separator comprising boehmite pigmentswith an average primary particle size of about 35 nm and cathodescomprising nickel-manganese-cobalt oxide (NMC). The small pore sizeprovides efficient diffusion of the small lithium ions (with theirassociated complex of organic carbonates or other electrolyte solventmolecules) with little to no loss in ionic conductivity. It was foundthat the diffusion of manganese and nickel ions from cathodes comprisingnickel-manganese-cobalt oxide (NMC) cathodes was inhibited from passinginto the separator and into the anode areas where undesired degradationof the battery chemistry could occur. The inhibition of the diffusion ofthe heavy and larger metal ions, such as manganese and nickel, could beseen, for example, by the absence of any coloration from these metalions in the white separators of the present invention. By comparison, nosuch inhibition was found with polyolefin and ceramic-coated polyolefinseparators where the ceramic coating had pores up to 500 nm or more indiameter.

If an increased diffusion inhibition is desired, the porous separatormay be treated with anionic, dianionic and other materials. Suchtreatments further reduce the pore size and provide charged species onthe walls of the pores, which may enhance the cationic nature of thealuminum or other inorganic element of the inorganic oxide or inorganicnitride of the separator. Such treated separators function to scavengeand remove HF and other degradation products, such as POF₃, of thelithium salt of the electrolyte, and increase the cycle lifetime, ascompared to known separators. Typically, the anionic compound is appliedto the separator from a solution comprising at least 2%, and preferably10% or more, of the anionic compound. The anionic compound may comprise0.1 weight percent or greater of the weight of the separator.

In one embodiment, the treatment of the separator may comprise acompound with only one anionic group, which complexes (or attaches) tothe aluminum or other cationic group of the separator and consequentlyhas no anionic group remaining free for complexing with any transitionmetal cations diffusing from the cathode. For example, the compound witha single anionic group could be an anthraquinone compound, such asanthraquinone-2-sulfonate (AQS). The AQS could be a sodium salt or,preferably, it is a lithium salt. Although the separator (aftertreatment with AQS or another monoanionic compound) does not have a freeremaining anionic group to complex with transition metal cations, it mayprevent the diffusion of transition metal cations into and through theseparator by the reduction of the pore size, particularly when theaverage pore size is less than about 50 nm in diameter and/or when theanionic compound is relatively large and in a planar shape, as is thecase with AQS and anthraquinone-2,6-disulfonate (AQdiS). Separatorstreated with monoanionic compounds, such as AQS, will still function asa photosensitizer, oxidizer of lithium metal, and oxygen scavenger aftertheir reduction, as described below for separators treated with AQdiSand other dianionic compounds.

Alternatively, the treatment of the separator utilizes a compound withtwo or more anionic groups. In such compounds, one of the anionic groupscomplexes to the aluminum or other inorganic cationic group of theseparator, while the other anionic groups remain free and available forcomplexing with any nickel, manganese, or other transition metal ionsfrom the cathode that may diffuse into the separator.

The anionic groups of the compounds with two or more anionic groupscomprise anions selected from the group consisting of sulfonates andcarbonates, and combinations thereof. The compound with two or moredianionic groups may include an anthraquinone compound, such as, forexample, AQdiS sodium salt or lithium salt. An aqueous solution ofAQdiS, particularly when heated to above 80° C. to provide a higherconcentration of AQdiS, readily complexes to a ceramic separator, suchas separators comprising boehmite. This complex is not dissolved by theelectrolytes used in lithium batteries and, thus, remains in the poresof the separator to act as an inhibitor to diffusion. When the pores ofthe ceramic separator are nanoporous, such as with an average pore sizeof less than about 100 nm, some of the sulfonate groups of dianioniccompounds, such as ASdiS, are sterically unable to complex to thecationic portion of the separator. This is beneficial because thesegroups of compounds remain free to complex with nickel, manganese, andother transition metal ions and reduce their undesirable diffusion intothe anode.

Some of the dianionic compounds suitable for use with the presentinvention, such as anthraquinone compounds with two or more dianionicgroups, have useful features in addition to reducing the diffusion oftransition metal cations into the anode. These additional usefulfeatures include, for example, functioning as a photoinitiator, and forreacting with and oxidizing any metallic lithium that contacts theseparator. AQdiS has a moderately strong absorption peak around 320 nmwith absorption out to about 400 nm and is an effective photosensitizerof UV-curing to photopolymerize monomers and oligomers. This attributeis useful in connection with the edge reinforcement procedure describedherein, as well as for improving the overall mechanical strength of theseparator. Using a hot water solution of AQdiS to treat the ceramicseparator, such as one comprising boehmite, may provide a stronglyabsorbing AQdiS photosensitizer with an optical density that is 0.3 orhigher than that of the separator before treatment.

It is generally understood that the safety of lithium batteries can beimproved by reducing or removing oxygen present in the electrolyte,since a lower oxygen content reduces flammability. In this regard, thephotosensitization by AQdiS involves the photo-induced reduction of theAQdiS, and typically the abstraction of a hydrogen atom from thephoto-sensitized compound. The transient, photo-reduced, AQdiSre-oxidizes in the presence of oxygen back to the original AQdiS. Thistype of reversible reaction of the photosensitizer is useful in removingoxygen from a system, such as in a battery electrolyte. Certainanthraquinone compounds, such as AQdiS, are reduced by contact withlithium metal and can function to oxidize any lithium metal dendritesthat come into contact with them. Since the reduced anthraquinonecompounds, in the presence of any oxygen, can be oxidized back to theoriginal anthraquinone compound, a single anthraquinone compound canreduce numerous lithium metal atoms over the lifetime of the battery,provided that there is oxygen available to oxidize the reducedanthraquinone compound.

The separators may also be calendered to further reduce their pore sizesto improve their inhibition of manganese and other large, heavy metalion diffusion, as well as to increase their mechanical strength. Forexample, a calendering that reduces the thickness of the separator byabout 8% was found to increase the tensile strength of the boehmiteseparator by about 15%.

In one embodiment, the separator is coated with a lithium metal layer to“prelithiate” the battery, such as, for example, a battery comprisingsilicon in the anode. In this regard, when a lithium-ion cell is chargedfor the first time, lithium ions diffuse from the cathode and areintroduced into the anode where they are reduced to lithium metal. As aresult, a decomposition product of lithium metal and the electrolyte,known as the solid electrolyte interface (SEI), readily forms on thesurfaces of the anode, wherein the thin SEI layer comprises lithium andelectrolyte reaction components. As the SEI layer is formed, a portionof the lithium introduced into the cells via the cathode is irreversiblybound and thus removed from cyclic operation, i.e. from the capacityavailable to the user. This process may consume about 10% to 20% of thecapacity of a lithium-ion cell and as much as 50% depending on theamount of silicon in the anode. It is therefore beneficial to“prelithiate” the anode (i.e., make more lithium available as the anodeactive material) in order to minimize the lithium consumption of thefirst charge-discharge cycle and thus the irreversible capacity loss.

Thermal runaway and other heat-related safety problems with lithium-ionand other lithium based batteries are well-known. Therefore, a thinsafety shutdown layer (not shown) may optionally be applied to theseparator 20 side of the coated stack. The safety shutdown layer rapidlyshuts down the operation of the battery when the temperature of the cellreaches a temperature in the range of 100° C. to 150° C., preferably inthe range of 105° C. to 110° C. In a preferred embodiment, this safetyshutdown layer has a thickness from 0.5 to 5 microns. The safetyshutdown layer coating may comprise water or alcohol solvents so that itcan be conveniently applied during the delamination, slitting, or otherconverting steps without requiring the use of a coating machine andinvolving undue mechanical stresses on the coated stacks without havinga release substrate attached. The safety shutdown layer may compriseparticles selected from the group consisting of polymer particles (e.g.,styrene acrylic polymer particles, polyethylene particles, andfluorinated polymers and copolymers) and wax particles, alone or inblends with each other.

Suitable safety shutdown layers are described in U.S. Pat. No. 6,194,098to Ying et al. Specifically, Ying teaches a protective coating forbattery separators (e.g., boehmite ceramic seperators) comprisingpolyethylene beads. See, e.g., Ying, Col. 10, 1. 66-Col. 14, 1. 19. Whena threshold temperature is reached, the polyethylene beads melt and shutdown the cell. Other suitable safety shutdown layers, particularly thosesuitable for use with both ceramic separators and other separators(e.g., plastic separators), are described in U.S. Pat. No. 9,070,954 toCarlson et al. Carlson describes a microporous polymer shutdown coating,see, e.g., Col. 2,1. 15-Col. 3,1. 28, that can be incorporated into thedisclosed coated stack and process.

As shown in FIGS. 2 and 3, one or more electrodes 40 a, 40 b are thencoated onto the separator layer 20. Suitable materials and methods forcoating electrodes directly on nanoporous separators are described in,for example, U.S. Pat. No. 8,962,182 (see, e.g., Col. 2,1. 24-Col. 3, 1.39, Col. 4,11. 49-56, Col. 5,11. 9-65 and Col. 6,1. 2-Col. 8,1. 7), U.S.Pat. No. 9,065,120 (see, e.g., Col. 3,11. 12-65, Col. 4,11. 18-61, Col.8,1. 2-Col. 9,1. 31, Col. 9,11. 42-67 and Col. 14,11. 6-23), U.S. Pat.No. 9,118,047 (see, e.g., Col. 2,1. 24- Col. 3,1. 33, Col. 4,11. 36-51and Col. 5, 1. 3-Col. 6,1. 21) and U.S. Pat. No. 9,209,446 (see, e.g.,Col. 2,1. 20-42, Col. 3,11. 1-56, Col. 5, 11. 16-31 and Col. 7,1. 1-Col.8,1. 65). These patents, as well as the applications referenced therein,are incorporated by reference in their entireties.

Depending on the requirements of the end use, the electrode coatinglayer 40 a, 40 b may be coated on the entire surface of the separatorlayer 20, in lanes or strips on the separator layer 20, or in patches orrectangle shapes on the separator layer 20. Cathode coating layers maybe coated from a pigment dispersion comprising water or an organicsolvent, such as N-methyl pyrrolidone (NMP), and contain theelectroactive or cathode active material in a pigment form, a conductivecarbon pigment, and an organic polymer. Anode coating layers may becoated from a pigment dispersion comprising an organic solvent or water,and contain the electroactive or anode active material in a pigmentform, a conductive carbon pigment, and an organic polymer. Theseelectrode pigments are particles with diameters typically in the rangeof 0.5 to 5 microns. Preferably, there is no penetration of theconductive and other pigments of the electrodes 40 a, 40 b into orthrough the separator layer 20.

In the embodiment shown in FIGS. 2 and 3 the electrodes are coated inlanes 40 a, 40 b. Electrode lanes 40 a, 40 b may be deposited using aslot die coater, or other methods known in the art. FIG. 2 shows anexample of a cross-section view of a portion of the assembly 1 followingthe coating of the electrodes 40 a, 40 b. FIG. 3 shows a plan view ofthe same assembly 1. Two lanes, 40 a and 40 b, are shown in FIGS. 2 and3 for ease of illustration. However, it should be understood thatadditional or fewer lanes, for example, 1-15 lanes (or even more), couldbe coated across the full width of the assembly in order to maximizeyield or volume output of the number of individual battery stacks thatcan be slit from the assembly.

In this regard, the electrode layer is coated in lanes 40 a, 40 b in adesired width for the final coated stack design and battery end use. Inone embodiment, the lanes 40 a, 40 b preferably have a width, W₁ of 12to 25 cm and are spaced apart from one another by a distance, W₂, of 2to 4 cm.

In one embodiment, shown in FIG. 4, a current collection layer 50 iscoated onto the electrode side of the assembly, which, at this point inthe process, comprises the substrate 10, release coating 30, separator20 and electrodes 40 a, 40 b. Methods of coating current collectionlayers, and materials for forming such layers, are disclosed inco-pending U.S. patent application Ser. No. 15/130,660.

For example, the current collector layer 50 can comprise nickel metal. Anickel current collection layer is preferred because it can be used as acurrent collection layer in either an anode stack or a cathode stack. Inaddition, nickel is generally less likely to oxidize and is moreelectrochemically stable than copper, aluminum, or other metals used incurrent collector layers. However, as discussed below, copper, aluminumand other materials can be used as well.

In order to improve the mechanical integrity of the coated stack,reinforcement areas 52 (shown in FIG. 4) may be added to the coatedstack. Reinforcement areas 52, preferably, cover the entire surface ofthe separator 20 between the electrode lanes 40 a, 40 b. Later in theprocess, the reinforcement areas 52 will become the edge or near edgeareas of the coated stacks when the stacks are slit to their finalwidth. The coating that comprises reinforcement areas 52 provides muchgreater mechanical strength to the coated stacks, especially for tearresistance and tensile strength. This is important after the coatedstacks have been delaminated from the strong and flexible releasesubstrate and have become free-standing. When they are free-standing,the coated stacks, especially the electrode layers, could (in theabsence of a reinforcement area) become brittle and may even crack ortear during processing. The presence of a mechanically strong andflexible edge reinforcement area 52 minimizes (and can even eliminate)the problem of tearing during the processes of delaminating, slitting,punching, tabbing, and stacking into the final cell. This approach ofedge reinforcement is also useful for free-standing separators, such asceramic separators.

In one embodiment, the reinforcement areas 52 are reinforced withpolymer. The polymer may be impregnated into the pores of the separator20 and/or coated over the separator 20. Alternatively, thisreinforcement could be provided by heating an overlying layer, such as aporous polymer safety shutdown layer (discussed above) to melt polymerin the edge areas of the separator into the pores or into a thin layeroverlying the separator. This approach includes a “sandwich”construction where the porous polymer layer, such as a shutdown layer,is between two layers of the inorganic separator. Upon heating the edgeareas, this trilayer construction is laminated and strengthened in theedge areas. Alternatively, the reinforcement of the edges of theseparator could be provided by utilizing the photosensitizing propertiesof the photosensitizer that is complexed to the separator (discussedabove). For example, during the converting process, a liquid comprisingphoto-curable compounds, such as 1,6-hexanedioldiacrylate (HDDA), couldbe coated in the edge reinforcement areas and then cured by UVabsorption by the photosensitizer in the separator. Additionalphotosensitizer can be added to the liquid comprising the radiationcurable compounds for additional curing efficiency and in the caseswhere the edge reinforcement is above the separator layer as well as inthe pores of the separator.

After coating to provide the current collector layer 50, a secondelectrode layer (not shown) can be coated onto the current collectorlayer 50. In a preferred embodiment, this second electrode layer iscoated in a lane of substantially the same width as the lane of thefirst electrode layer 40 a, 40 b and directly over the position of thefirst electrode layer. This provides anode and cathode stacks with anelectrode coating on both sides of the current collector, which are themost typical cell assembly configuration for the electrodes, i.e.,double side electrode coatings on the current collector layer. After thesecond electrode coating, the coated stack on the release substrate ispreferably calendered to densify the second electrode.

Next, the assembly is prepared for tabbing, i.e., electricalinterconnection. In the embodiment shown in FIG. 5, patches 60 of aconductive material have been coated in the desired tabbing location toobtain high electrical conductivity in these areas. Patches 60 are inelectrical contact with current collector 50. It should be understoodthat the placement and number of conductive patches 60 will vary basedupon the particular battery design. As will be discussed further below,the embodiment shown in FIG. 5 represents a patch 60 configuration for acylindrical or “jellyroll” layout.

In one embodiment, the next step is to delaminate the coated batterystacks from the release substrate 10 so that the coated stacks may beconverted into finished cells. As discussed above, to save cost, thesubstrate 10 may be re-used for making another coated stack. Preferably,the release substrate 10 is cleaned and inspected prior to each re-use.

The next step is to slit the coated stack assembly 1 to the desiredwidth. In the embodiment shown in FIG. 5, slitting is done through theareas of the separator layer 20, namely S₁, S₂ and S₃, which do notcontain electrode or current collector layers. Since the separator layer20 and reinforcement areas 52 are the only layers slit, there is nopossibility of generating conductive fragments or debris, e.g., from theelectrode or current collector layers. By comparison, in prior artmethods, slitting is typically performed through a metallic orconductive metal foil layer. However, slitting these metal layersgenerates conductive debris (e.g., metal shards or shavings) that cancause the cell to fail during manufacture or in the field due to a shortcircuit, which can result in a fire or the explosion of the battery.Thus, the potential for such dangerous conditions are avoided with thepresent invention.

The embodiment shown in FIG. 6 provides a coated stack 70 for use in ajellyroll configuration. In this regard, the coated stack 70 would bewound with a coated stack of the opposite polarity into a jellyroll andpackaged in a cylindrical case. The discrete coated stacks 70 can betabbed and welded using conventional methods.

Now that the preferred embodiments of the present invention have beenshown and described in detail, various modifications and improvementsthereon will become readily apparent to those skilled in the art. Thepresent embodiments are therefor to be considered in all respects asillustrative and not restrictive, the scope of the invention beingindicated by the appended claims, and all changes that come within themeaning and range of equivalency of the claims are therefore intended tobe embraced therein.

What is claimed is:
 1. A lithium battery comprising: an anode, acathode, wherein the cathode comprises one or more transition metals, anelectrolyte, and a porous separator interposed between the cathode andanode, wherein the porous separator comprises boehmite and an anioniccompound comprising sulfonate groups, wherein the anionic compound is ananthraquinone.
 2. The battery of claim 1, wherein the anode compriseslithium metal.
 3. The battery of claim 1, wherein the cathode comprisesone or more transition metals selected from the group consisting ofmanganese, nickel and cobalt.
 4. The battery of claim 1, wherein theporous separator comprises a reinforcement area along an edge of theporous separator and wherein the reinforcement area comprises a polymer.5. The battery of claim 4, wherein the reinforcement area comprises apolymer impregnated in the pores of the porous separator.
 6. The batteryof claim 1, wherein the anionic compound comprises two or more sulfonategroups.
 7. The battery of claim 6, wherein a cation of the anioniccompound comprises a lithium ion.
 8. The battery of claim 1, wherein theanionic compound comprises greater than about 0.1 weight percent of theweight of the porous separator.
 9. The battery of claim 1, wherein theanionic compound is a photo sensitizer.
 10. The battery f claim 9,wherein the photosensitizer is an oxygen scavenger.
 11. The battery ofclaim 9, wherein the separator comprises polymer formed by theabsorption of photons by the photosensitizer.
 12. The battery of claim1, wherein the anionic compound is an oxidizer of lithium metal.
 13. Thebattery of claim 1, wherein the porous separator has an average porediameter of less than about 100 nm.
 14. The battery of claim 1, whereinthe porous separator is a scavenger of HF in the electrolyte.
 15. Thebattery of claim 1, wherein the porous separator inhibits the diffusionof transition metal cations through the separator.
 16. The battery ofclaim 1, wherein the cathode and the anode comprise electrode layers andone or more of the electrode layers are coated on the separator.
 17. Thebattery of claim 1, wherein the suifonate groups are complexed to theboehmite.